The present invention generally relates to the extrusion of polyolefins, particularly low density polyethylene, linear low density polyethylene and high molecular weight, high density polyethylene resins in the form of film, especially blown or cast film or sheet.
Linear low density polyethylene (LLDPE) resins were introduced in the U.S. market recently and have had a substantial impact on industry. The mechanical properties of LLDPE provide superior performance, in terms of strength and puncture resistance, which results in thinner films useful in a variety of applications and have resulted in the opening of new markets.
LLDPE, due to its unique molecular structure and resulting rheology, has created special problems in processing. These problems include the net reduction of throughput due to the high horsepower required to process a kilogram of polymer and the requirement for more torque than is available for most blown film drives to maintain equivalent throughput. Since more torque is needed to extrude LLDPE, extruders are pushed to the limits of their capabilities and the resulting melt temperature is higher than desired for extruding blown film. The higher melt viscosity and higher temperatures are detrimental to the machinery and to the end product.
To maintain throughput, modification of equipment, modification of the resin and the use of higher temperatures, or a combination thereof, are required. Higher torque, higher temperatures and higher pressures have detrimental effects on the properties of the final products, including the phenomenon of sharkskin melt fracture, i.e., surface roughness of the film. Sharkskin is largely a result of high melt temperature from frictional heat generated by higher extrusion torques.
The extrusion of LLDPE requires substantially higher power for processing than low density polyethylene (LDPE). In addition, high and ultra high molecular weight, high density polyethylene (HDPE) require additional power to extrude and their torque requirements can be double that of LDPE. Most extruder drives do not have the capability of generating such high torque and, therefore, extrusion throughput is sacrificed to accommodate the higher torque requirement.
The reduction in screw speed which is necessary to compensate for mechanical overloading of the extruder and its critical components, such as gear box and thrust bearings, creates yet another major problem for the film processor. The rotational speed of the screw in the extruder is a function of drive power; reduction in screw speed as a result of lower torque results in lower throughput.
Throughput studies indicate that a conventional LDPE extruder which would normally handle a 2.0 melt index (MI) LDPE, suffers a 27% loss in throughput when a 1.0 MI LLDPE is processed.
The loss of throughput has created a market for the manufacturers of machinery and equipment capable of extruding LLDPE without throughput loss. Many screw designs have been suggested and evaluated. In fact, the so-called "short screws" are being used in applications where LDPE blown film extruders have been retrofitted for blown LLDPE film production. These modifications of existing machinery or purchases of new generation extruders involve large sums of capital. In the majority of cases, such an investment is impractical.
Present manufacturers are also trying to change the processing requirements of LLDPE resins by modification of resin technology.
Another approach is to blend LLDPE with LDPE resins. Such blending to increase production and ease processing requirements has partially solved some of the problems associated with the extrusion of LLDPE resins. Depending upon the ratio of LDPE to LLDPE in the blend, the properties of the blended resin film are substantially different from that of unmodified LLDPE film as evidenced by inferior draw-down capabilities and other physical properties.
LLDPE film exhibits superior tensile strength, tear strength, draw-down characteristics, stiffness and puncture resistance. These properties, however, will dramatically deteriorate as the level of LDPE in the blend increases. The two most important properties of LLDPE, film tear resistance and puncture resistance, will decrease considerably with the increase of LDPE in the blend.
Accordingly, it is an object of the present invention to provide a processing aid composition which is blended with polyolefins, especially with LLDPE or LDPE, or blends thereof, or HDPE resins or blends thereof to afford increased extrusion throughput by altering the rheology of the resins, thereby resulting in higher screw speeds, lower head pressures, reduced power consumption, lower torque and lower processing temperatures while maintaining throughput.
Additionally, it is an object of the present invention to provide a composition comprising a polyolefin or polyolefin blends, particularly LLDPE or LDPE resins, or blends thereof, or HDPE and blends thereof and a processing aid composition therefore.
Further, it is an object of the present invention to provide a process for extruding polyolefins such as LLDPE, LDPE or HDPE and blends thereof which comprises adding to said polyolefin, prior to extrusion, a processing aid composition as described herein.